In gas turbine engines, for example, aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted rotary compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on a shaft. The flow of hot gases turns the turbine, which turns the shaft and powers the compressor. The hot exhaust gases flow from the back of the engine, providing thrust that propels the aircraft forward.
Gas turbine engines generally include a high pressure compressor for supplying combustion air to a combustor, and a turbine. The high pressure compressor, combustor, and turbine are collectively referred to as a core engine. Typically, gas turbine engines also include a low pressure compressor for supplying compressed air, for further compression, to the high pressure compressor, and a fan for supplying air to the low pressure compressor.
These compressors typically include a plurality of stages, each stage including in alternating configuration a rotor for moving the air axially and a fixed, radially oriented stator for efficiently directing the flow of air axially. The rotor typically includes an assembly of a plurality of blades radially attached to a rotating disk, the assembly surrounded by a casing. The casing is typically fabricated to be removable, such as by forming the casing into two halves that are then removably joined together. The casing supports the plurality of radially oriented, fixed stator vanes which are attached thereto, while the rotor supports the rotor blades. Each stage of stator vanes are positioned in front of a rotor with the attached blades to efficiently direct air flow to the blades of the rotor.
Variable stator vane assemblies are utilized to improve the performance of the engine. For better performance, the rotational speed of the fan and compressor usually need to be different. In general, the high speed compressor rotates about twice as fast as the fan. This is accomplished by attaching the compressor and fan to different spools or shafts which run concentric to each other. In this dual spool configuration, the high pressure compressor is connected to a high pressure turbine by an outer spool. In some configurations, three concentric spools are utilized. Each variable stator vane assembly includes a variable stator vane which extends between adjacent rotor blades. The variable stator vane is rotatable about a substantially radial axis. The orientation of the variable stator vane varies the stagger angle of the vane in a controlled fashion. This allows the vane or vanes to be realigned to change the impingement angle of compressed air on to the rotor blades as the operating condition of the engine changes. The position of the vane is changed by means of a lever arm attached to an actuator ring on the outside of the compressor case.
A known variable vane assembly includes a variable vane, a trunion bushing; and a washer. At an outer end, the variable vane assembly is bolted onto a high pressure compressor stator casing and the bushing extends concentrically through an opening in the casing. The washer is positioned above the casing and between the bushing and casing. The variable vane includes a vane stem that extends through the opening in the casing (hereinafter referred to as the “outer end”) and through the bushing and washer. The bushing and washer are referred to herein as a bearing assembly, the bearing assembly positioned radially outboard referred to as the first bearing assembly. The vane also includes a second bearing assembly at its inner radial end. The vane may be shrouded at its inner end to minimize the vibrational effect of flow variations, particularly on the longer vanes. The bearing assembly produces a low friction surface that prevents metal on metal contact.
A lever arm is fixedly joined to the vane stem extending outwardly from the vane bushing or first bearing assembly. The distal end of the lever arm is operatively joined to an actuation ring that controls the angle of the vane. All of the vane lever arms in a single stage are joined to a common actuation ring for ensuring that all of the variable vanes are positioned at the same angular orientation relative to the airflow in the compressor stage.
Although known variable vane assemblies provide certain advantages as explained above, such vane assemblies have potential gas stream leakage paths which reduce engine efficiency. The primary leakage path is between the outside diameter of the airfoil portion, the stator vane stem, extending through the aperture in the compressor casing and the inside diameter of the bushing. The secondary leakage path is between the outside diameter of an optional metal jacket housing a portion of the bushing or alternatively, the bushing itself and the inside diameter of the aperture opening in the compressor stator casing. Other leakage paths are on either radial end at the airfoil where the airfoil joins the case and the shroud, as well as at the shroud seal, between the shroud and the rotor shaft. Additionally, the high velocity and high temperature of the air can cause oxidation and erosion of the bearing assemblies, which leads to premature failure of the bearing assembly, and eventual inability of the variable vane assembly to function. This will decrease engine efficiency and ability to rapidly respond to power demand changes.
Once the bearing assembly fails, an increase in leakage through the opening occurs, which results in a performance loss. In addition, failure of the bearing assembly can allow contact between the stator vane and the casing, which causes wear as a result of vibration and increases overhaul costs of the engine. Accordingly, it would be desirable to provide bearing assemblies fabricated from materials and of a design having performance characteristics that will reduce or eliminate air leakage between the stator vane stem and the compressor casing while providing an increase in the durability of the bushing and washer composition to increase part life. The present invention fulfills this need, and further provides related advantages.